Repeated shutdown command should be accepted

Current Situation

If a shutdown command has been sent to the agent, any further shutdown commands will be ignored.

Desired Behavior

The shutdown command can optionally send a signal (SIGTERM, SIGKILL) to the jobs still running on the agent.

Repeated shutdown commands should be accepted if they accelerate the shutdown process, i.e.

• Shutdown command without signal allows another shutdown command with signal
• Shutdown command with SIGTERM allows another shutdown command with SIGKILL